The present invention relates generally to an apparatus and method for maintaining a steady state current in a plasma for magnetically confining the plasma in a toroidal magnetic confinement plasma device such as a tokamak. More particularly, the present invention relates to a method and apparatus for maintaining a steady-state current for magnetically confining the plasma in a toroidal magnetic confinement device using anomalous viscosity current drive. A second aspect of this invention relates to an apparatus and method for the start-up of a magnetically confined toroidal plasma.
The apparatus for toroidal magnetic confinement that is most popular in controlled fusion research today is the tokamak device. But, as is widely recognized, the standard tokamak is inherently a pulsed devise. The magnetic-field-aligned, predominantly toroidal, plasma current, essential for tokamak plasma confinement, is, in the standard tokamak, driven by electromagnetic induction. However, a number of means have been proposed to enable steady-state tokamak current drive. To date, the most successful of such means is lower-hybrid radio-frequency current drive, but the efficiency of radio-frequency current drive is projected to be marginal for a fusion reactor. The present invention pertains to a method and apparatus for driving a steady-state current in a magnetically confined plasma, such as a tokamak plasma, that may prove to be of superior efficiency for a fusion reactor.
For a classical axisymmetric magnetically confined toroidal plasma, there is no mechanism within resistive magnetohydrodynamic (MHD) theory to balance the electron-ion friction associated with plasma of current flow other than the presence of a toroidal electric field. But the latter can be established inside the tokamak plasma only by magnetic induction, a process inconsistent with steady-state operation. If steady-state current drive is to be created, it must involve forces which do not appear in the usual Ohm's Law, such as injected high-Z ions, or resonant electrons (radiofrequency drive), or mean-field terms that have their origin in non-axisymmetric phenomena. Anomalous viscosity for plasma current flow is just such a non-axisymmetric phenomenon and can be described in mathamatical terms by adding just such a mean field term to Ohm's Law.
In more detail, Furth et al., Phys. Fluids 16, 1054 (1973), have demonstrated that a strong current on the edge of a plasma will trigger the double-tearing instablilty, provided the direction of flow of this edge current is parallel to that of the interior plasma current. The process of magnetic reconnection or of magnetic turbulence associated with this instability then facilitates the rapid radial penetration of the properly directed edge current. Rapid current penetration due to the double-tearing instability has been suggested by Furth et al. (op. cit) and by Stix, Phys. Rev. Letters 36, 521 (1976). This penetration can be modeled by adding to Ohm's Law a new term describin,g the meanfield behavior of the instability-driven turbulence. The effective shear viscosity associated with the effect of magnetic turbulance or of magnetic reconnection on magnetic-field-aligned plasma current flow will hereinafter be referred to as anomalous shear viscosity for current flow or more simply, just as anomalous viscosity. For a detailed discussion of this model a reference is made to Stix, Nucl. Fusion 18, 3 (1978).
Ono et al., Bull. Am. Phys. Soc. 27,967 (1982), demonstrated in an experiment performed on the ACT-1 plasma facility that an electron beam produced by an electrode system could be used to produce a highly ionized plasma. It should be understood, however that this ACT-1 experiment used the electrode system in a substantially different manner than the electrode system used in the present invention, as discussed below in DETAILED DESCRIPTION OF THE INVENTION.
Therefore, in view of the above, it is an object of the present invention to provide an apparatus and method for maintaining a steady-state plasma current in a toroidal magnetically confined plasma.
It is another object of the present invention to provide an apparatus and method for maintaining a steady-state current in a toroidal magnetic confinement plasma device, the apparatus and method having a higher efficiency than radiofrequency current drive.
It is another object of the present invention to provide an apparatus and method for maintaining a steady-state plasma current in a toroidal magnetic confinement plasma device using anomalous shear viscosity to achieve preferential penetration of properly-directed magnetic-field-aligned plasma current.
It is still another object of this invention to provide an apparatus and method for creating a toroidal magnetically confined plasma, ab initio, in a toroidal magnetic confinement device.
Additional objects, advantages and novel features of the invention will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.